This MySQL error has encountered when the temporary directory is full. MySQL often creates temporary tables for some queries and most of them are placed in tmpdir. Which may require enough free disk space available for MySQL.
This is a MySQL error and nothing to do with WordPress files. To resolve this problem we can check with these options:
1) Repair database table from PHPMyAdmin.
2) You are out of space on /tmp or you have too many files in /tmp even if there is lots of free space. In this case you need contact your hosting service provider to resolve this issue.